Password lock files on iPod Touch?

Anyone know of a way (or a functioning ap) to lock individual files on the Touch? I've recently received one for my classroom and I have different files that different periods will need. I'd like to keep them focused on the required task instead of playing around with it.

I know I can password lock the whole device but can I do so for individual files (or even better pages?)

I know the best option is to teach kids to use them responsibly and I have no problem with that but I'd sure like to be able to speed up the process with a little password encouragement!